Measurement systems such as signal analyzers and spectrum analyzers are used in a variety of applications to measure various types of signals under various types of conditions. In aerospace and defense applications, the task of signal measurement may be used to search for known or unknown signals across a broad frequency spectrum. In wireless communications, there is a need to characterize increasingly complex devices in an ever-increasing number of conditions and device states as quickly as possible. In all cases, searching for signal images and spurs requires instrumentation that can locate low-level signals at unknown frequencies across a wide frequency range. Making such measurements requires a spectrum analyzer or a signal analyzer (referred to collectively hereinafter as a “signal analyzer”) that can perform high-speed measurements with a low noise floor, minimal spurs and images, and high dynamic range.
Signal analyzers generally fall into two broad categories, namely, preselected signal analyzers and non-preselected signal analyzers. A preselected signal analyzer has preselection circuitry that performs a preselection algorithm that removes images and spurs from an intended radio frequency (RF) signal that is being measured. A typical preselected signal analyzer may not, however, be fast enough for high-speed, high-resolution measurements. In non-preselected signal analyzers, the preselection algorithm is performed in software, and thus the preselection circuitry is eliminated. By eliminating the preselection circuitry and performing the preselection algorithm in software, non-preselected signal analyzers can provide advantages in terms of speed, accuracy and noise floor compared to preselected signal analyzers. However, the preselection algorithm that is implemented in software in a non-preselected signal analyzer must be sufficiently robust to remove unwanted images and spurious mixer products.
The preselection algorithm that is performed in software typically involves, in its simplest form: sweeping or stepping a frequency of a local oscillator (LO) of the signal analyzer through a first frequency range that has a lowest frequency that is less than the frequency of the intended RF signal to be measured (low-side mix) and that includes the frequency of an intended RF signal; measuring the input RF signal to obtain a reference trace; shifting the frequency range of the LO higher by twice the intermediate frequency (IF) to a second frequency range (high-side mix); sweeping or stepping the frequency of the LO through the second frequency range and again measuring the input RF signal to obtain an alternate trace; performing a mathematical operation that processes the reference trace and the alternate trace to remove any images from the measured RF signal; and displaying the final trace from which images have been removed.
The operation that processes the reference trace and the alternate trace to remove any images from the measured RF signal is typically referred to as a min-select operation. In general, where the reference trace and the alternate trace agree with one another, the response is real and where they disagree with one another, the disagreement is an image that should be rejected. Because images only add to the response, the images can be removed by comparing the minimum values of the reference trace and the alternate trace at each point, or frequency increment, and selecting the minimum of the two values as the value to be used in the final trace that is displayed.
The disadvantage of the preselection algorithm described above is that using the minimum of the two values introduces a bias in the final signal for cases where the input signal being measured is a noise-like signal. In essence, the min-select operation will affect the statistics of the measured noise and bias the measurement results. One solution to reduce the negative bias is to average each of the alternate traces over time before performing the min-select operation. This solution reduces, but does not eliminate, the bias.
One known solution for eliminating the bias, which is not entirely effective, is to use a user-selected, threshold value during the min-select operation. With this approach, the minimum values of the reference trace and the alternate trace at each point, or frequency increment, are compared and the minimum of the two values is then compared to the user-selected threshold value. If the minimum value is above the threshold value, then the minimum value is used in the final trace that is displayed; otherwise the reference trace value is used in the final trace.
A disadvantage of this solution is that the selection of the threshold value is left up to the user, who typically does not know how to select a suitable threshold value. If the threshold value is set too high, images at lower decibel levels will fail to be rejected. If the threshold value is set too low, some of the traces will have sufficient variability that the bias problem will not be eliminated.
A need exists for a non-preselected signal analyzer that is capable of performing the preselection algorithm in a manner that eliminates the aforementioned bias problem and that is robust, accurate and capable of being performed at high speed to meet the needs in the industry for measuring high-speed signals.